Sudden cardiac death in patients with coronary artery disease may be due to development of sustained irregular heart rhythm (ventricular arrhythmia) during episodes of myocardial ischemia (reduced blood supply to heart muscle), which may otherwise be completely silent. Using a special method whereby the electrocardiographic signal can be averaged over several beats, so as to reduce the amount of noise, and increase the resolution in the terminal part of the QRS complex, it is possible to detect patients with late potentials. The presence of late potentials during signal-averaged electrocardiography is associated with an increased incidence of re-entrant ventricular arrhythmias. In order to determine whether silent episodes of myocardial ischemia can create a substrate within the heart muscle which will allow for development of irregular heart rhythm, we performed a study where signal-averaged electrocardiograms were performed immediately after treadmill exercise testing in patients with coronary artery disease who develop myocardial ischemia with exercise and control patients with normal coronary arteries. Only one patient had development of late potential after treadmill exercise testing, which resolved with rest. Such an abnormality was not present in normal volunteers. These findings suggest that exercise-induced transient myocardial ischemia does not promote a myocardial substrate for ventricular arrhythmias in most cases. However, in less than 5% of cases this may occur and may be a cause of sudden cardiac death during silent myocardial ischemia in that subgroup.